Totally think for yourself. My mom used to say GOD knows what's in your heart and to beware of hypocrites. Idk what you know, but I definitely enjoy this conversation.
Angel is a messenger of GOD. Angels visited Mary and Joseph. I've no idea how many there are, but as a human we can't actually comprehend what space even is. Angels aren't supposed to be worshiped so we don't focus on their names, or them too much. There's a whole host who simply praise GOD and his glory shouting "HOLY HOLY HOLY". Like if you get shocked by something and you say "holy shit", like that but so immense that all they can say is HOLY.
Fallen are those 1/3 of the heavenly host who rebelled, lost, and then were cast down. The war of Lucifer vs Michael.
No, once they leave GOD/fall they are referred to as demons. They're in darkness. Their sin took place in eternity so they can't be forgive or expunged for it. We are mortal so we can die and leave that life behind. Hence why and how GOD still has domain over them.
Lucifer was the angel, the most beautiful. GOD loved him, and said he was perfect. But inequity was found in him, and Satan is the same being after falling. Satan was proud and wouldn't bow to humans, therefore disobeyed GOD. Though you can argue, oh after that decision are you the same? But yeah same thing, but different times. Like how Saul's name was changed to Paul. Simon became Peter. Names have meaning and they change once you become that new thing.